Thymic stromal lymphopoietin (TSLP) is an important factor involved in the pathogenesis of asthma and allergy. Mucosal tolerance is induced early in life and is an important mechanism of protection from allergic diseases such as asthma. Our preliminary results demonstrate that neonatal CD4+ T cells are tolerogenic and, even when immunized with OVA+Alum, preferentially differentiate into inducible regulatory T cells (iTregs) rather than T helper 2 (Th2) cells. We find that infants (age range 6-18 months) with atopic dermatitis (AD) have elevated serum TSLP (200 - 300 pg/ml), a concentration sufficient to suppress iTreg differentiation. Failed to establish airway tolerance against a particular aerosol antigen, the individuals may become sensitized at a later time in the presence of natural adjuvants such as microbial products in the environment. Recent studies show that majority household dust extracts contain bacterial-derived glycolipids capable of activating human and mice NKT cells and thus act as natural adjuvants to promote Th2 sensitization against aerosol allergens. We surprisingly find that this glycolipid-induced Th2 sensitization requires TSLP, and the mechanism underlying this requirement is unknown. Taken together, we hypothesize that skin-derived TSLP in young infants disrupts the establishment of tolerance in airway mucosa, rendering them more susceptible to be sensitized against environmental antigens in the presence of natural adjuvant-induced TSLP, the two-step mechanism underlying the atopic march. Thus, we will test our hypothesis in two Specific Aims. 1) Define the role of skin-derived TSLP in the regulation of airway tolerance. 2) Define the role of TSLP in NKT cell activation-mediated Th2 polarization in airway mucosa. The information learned from these studies will provide a greater understanding of the role of TSLP in asthma pathogenesis, and insights in targeting TSLP to prevent atopic march.